As others have said, no antimalware product will be 100% effective 100% of the time. If there was such a creature we'd all be using it!
Some other considerations include:
1. What are the specifications for the specific computer (like CPU, RAM, etc.) Some antimalware programs are more "bloated" than others and could slow a machine down.
2. What programs (software) are installed and will they play nice with the chosen antimalware product?
3. What are the individual requirements of the user(s)? Will the machine be used to download torrents? Visit questionable sites? In other words, even a top rated antimalware program could be pushed beyond its limits.
Since we're talking about free products, there's nothing better than to give them a try. Even the paid products usually have a free trial period. Put each product to the test based on your friend's real life, day-to-day activities. CAUTION: Make sure your friend creates a system image (not just a restore point.) That way, when the computer becomes infected he/she will be able to revert back to a clean image without having to manually reinstall everything on the hard drive.